1. Field of the Invention
The present invention relates to a table apparatus employing linear motors as driving means and capable of operating both in fine-feed mode and large-feed mode.
2. Description of the Prior Art
The present inventor has already proposed a table apparatus having the following features, in the specification of Japanese Patent Appliciation No. 198803/1983 which was filed in advance of the present application but not laid-open yet. Briefly, this table apparatus has a movable table movably supported through linear bearings on a track member which in turn is mounted on a stationary bed. The stationary bed carries, rotatably but not axially movably, a ball screw shaft which in turn carries, through steel balls, a ball nut for free movement both in the rotational and axial directions. The ball nut is suitably connected to the movable table. A ball spline shaft is rotatably mounted on the stationary bed, in the vicinity of the ball screw shaft, and an outer sleeve is splined, through the intermediary of steel balls, to the ball spline shaft for rotation therewith and for axial movement thereon.
The outer sleeve is operatively connected to the ball nut through a suitable connecting mechanism. The ball screw shaft and the ball spline shaft are driven by respective rotary motors, so that the speed of feed of the movable table is varied from fine feed speed to large feed speed in a stepped manner by a suitable selection of the direction of operation of the rotary motors.
This table apparatus employing the ball screw and nut assembly and the ball spline assembly, however, suffers from a problem in that the response of movement of the movable table is impaired, to make it difficult to precisely control the position of the movable table, due to undesirable twisting of the screw shaft and the spline shaft by the driving torque and also due to the presence of small backlash or play inevitably existing between the screw shaft and the nut and between the spline shaft and the outer sleeve.
In addition, the moving table carrying heavy parts such as the ball screw and nut assembly and the ball spline assembly exhibits a large inertia which undesirably impairs the starting and stopping response characteristics.
It is to be pointed out also that the size of the table apparatus as a whole is increased because of the necessity of the spaces for mounting the rotary motors, ball screw and nut assembly and the ball spline assembly.